new guy here.. my 3 yo son now has 4 motorized vehicles and i think that means we are at the early stages of a serious hobby.. anyway i have read a little and decided its best to charge the batteries with a schumacher charger instead of the factory supplied wall adapters. i got the small 1.5A maintainer and when i hook it up i hear a sizzling sound from the batteries.. i tried two different batteries.. both are 6V and the charger works for 6 or 12 batteries. voltage i measure is about 7.4. quickly googling the subject it doesnt sound like the batteries should be making sounds if they are charging at an appropriate rate. can anyone confirm?

factory batteries.. fisher price 9ah and peg perego/kung long 4.5ah. both were literally freezing.. just pulled them out of the garage in 20some degree weather. they didnt get warm and no sign of swelling. the sizzle sound settled down after a short time.